The influence of fatty acids on tribological and thermal properties of natural oils as sustainable biolubricants
Introduction
Pure natural oils have been known to be good lubricants since ancient times in lowering friction and preventing wear. In 1859, the first commercial oil well was drilled in Titusville, PA, USA. This led to the rise of the modern petroleum oil industry, which would eventually decline the use of pure natural oils as lubricants. The advent of petroleum-based oils produced rapid advances in lubrication technology that quickly dominated other oils, such as natural oils in the lubrication industry. During the mid-1930s, the properties of petroleum-based oils were significantly improved through the use of additives in the oil to enhance the load carrying capacity, lubricity, corrosive protection, and thermal-oxidative stability. These improvements in properties of petroleum-based oils often surpassed similar properties of natural oils [1].
Since the beginning of the 20th century, investigations into the properties of natural oils have received much attention [2], [3], [4], [5], [6], [7], [8]. This revival is due to the fact that 50% of all lubricants worldwide end up in the environment through usage, spill, volatility, or improper disposal [9], [10]. Many of these lubricants entering the environment are derived from petroleum-based oils and are deleterious to sensitive biological ecosystems. More recently, in an effort to curb the use of petroleum-based lubricants due to concerns of protecting the environment, depletion of oil reserves, and increases in oil price, natural oils have witnessed a resurgence. Furthermore, the emphasis placed on natural oils is a result of the increase in demand for environmentally-friendly lubricants that are less toxic to the environment, renewable, and provide feasible and economical alternatives to traditional lubricants [11].
Recently, the industrial market has been shifting to become more ecologically responsible with much of the attention focused on energy conservation and sustainability through the use of natural oils for industrial purposes. A primary focus has been on the development of technologies that incorporate plant oils as biofuels and industrial lubricants because they are non-toxic, biodegradable, and renewable [12]. In the lubrication market, the term “biolubricants” refers to all lubricants derived from bio-based raw materials (plant oils and animal fats), which are biodegradable and non-toxic to humans and other living organisms, particularly aquatic environments where the impacts are more detrimental. Many biolubricants are comprised of plant oils, animal fats, or chemical modifications of these oils and are widely regarded as environmentally benign because of their superior biodegradability and renewable feedstock [13]. Moreover, many bio-based oils have superior lubricity and wear resistance that exceeds similar properties of petroleum-based oils resulting in the increased usage of biolubricant base-stock for industrial oils and functional fluids, thus ‘fueling’ a biolubricant resurgence [14].
Natural plant oils have a higher lubricity, lower volatility, higher shear stability, higher viscosity index, higher load carrying capacity, and superior detergency and dispersancy when compared to mineral and synthetic oils [3], [4]. Many of the accolades associated with natural oils are a result of their molecular structure, which affords better lubrication properties. The attraction to natural oils is due to their chemical composition of triacylglycerol molecules made up of esters derived from glycerol and long chains of polar fatty acids. The fatty acids are desirable in boundary lubrication for their ability to adhere to metallic surfaces due to their polar carboxyl group, remain closely packed, and create a monolayer film that is effective at reducing friction and wear by minimizing the metal-to-metal contact [4], [5], [6], [7], [8], [15], [16], [17], [18]. Much of the work with bio-based oils has concentrated on understanding the fundamentals of saturated and unsaturated fatty acids with the bulk of the attention focusing on the use of natural oils as neat lubricants, fatty acids as additives in mineral oils, and bio-based feedstock oils for chemically-modified lubricants [3], [11], [19], [20], [21].
Despite these favorable attributes, there are a number of drawbacks to bio-based oils including their price (costing three times more than traditional lubricants), poor thermal-oxidative stability, solidification at low temperatures (high pour points), biological (bacterial) deterioration, and hydrolytic instability (aqueous decomposition) [18], [22], [23], [24], [25], [26], [27]. Additionally, many biolubricants are susceptible to rapid oxidative degradation due to the presence of free fatty acids and double bonds in the carbon chains of the triacylglyceride molecules. Previous research [28], [29] indicates that thermal-oxidative stability of natural oils requires a low percentage of polyunsaturated fatty acid (i.e. linoleic acid). Thus, the oxidative stability increases with decreasing amounts of polyunsaturated fatty acids. Furthermore, monounsaturated fatty acids such as oleic acid having one double bond improve oxidative stability while simultaneously providing good low temperature properties and superior tribological properties. The best compromise between thermal-oxidative stability and low temperature properties are through the use of naturally high oleic acid oils or by genetically modifying the base-stock of low oleic acid oils to yield high oleic acid oils, such as high oleic acid safflower oil (HOSO), canola oil, sunflower oil, or soybean oil all of which are commercially available and derived from genetically modified organisms (GMOs) [17], [18].
Although biolubricants and bio-based functional fluids have their shortcomings, they have many uses ranging from basic lubrication to transmission of energy to protection against corrosion and wear (attrition) and to the removal of heat, wear debris, and foreign contaminants [30]. Recently, natural oils are finding uses as carrier fluids for lamellar particle additives in sliding contact [2], [4], [31], [32], [33]. The efficacy of natural oils is determined by their fatty acid composition, which affects their specific properties allowing them to be used as lubricants, fuels, and functional fluids. The sources of bio-based oils are numerous and encompass a wide variety of vegetables, nuts, fruits, animals, and marine sources. Inherently, bio-based oils are naturally occurring organic substances primarily composed of triacylglycerol whose properties and utility vary based on biological factors such as nutrient availability, climate, light, temperature, and water, which can influence their tribological properties [34]. Notwithstanding their inconsistent and widely variable chemical composition, biolubricants remain to have superior wear resistance, lower coefficients of friction, lower volatility, higher viscosity index, excellent biodegradability, higher flashpoints, sustainable and renewable feedstock, and a lower ecotoxicity classification than mineral oils [35]. Furthermore, research has focused on thwarting the deficiencies of natural oils and other biolubricants while seeking to understand the relationship between chemical composition, molecular structure, and properties through chemical modifications such as epoxidation, metathesis, acylation, estolide formation, transesterification, and selective hydrogenation of plant oils with polypols, and additivation [17], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46].
In this study, several natural oils were selected to represent a broad range of saturated and unsaturated fatty acid compositions within bio-based oils. These oils were investigated to understand the effects of fatty acid composition on friction and wear performance under ambient conditions using a pin-on-disk apparatus. Further, a thermogravimetric analysis was used to determine the correlations between the fatty acid composition, tribological performance, and the thermal response of the natural oils.
Section snippets
Natural oils
To investigate the tribological performance of natural oils, eight oils were selected: avocado, canola (rapeseed), corn, olive, peanut, safflower, sesame, and vegetable (soybean) oil. These oils were chosen because they represent a variety of saturated, monounsaturated, and polyunsaturated fatty acid compositions within natural oils. The natural oils are derived from a broad range of bio-based feedstocks and are readily available and inexpensive. Additionally, they have viscosity and surface
Friction analysis
Fig. 2(a) depicts the variation of the coefficient of friction with sliding distance for various natural oils under ambient conditions. It can be seen that a general trend exists where there is a decrease of the coefficient of friction (COF) significantly at the start of each experiment. The COF of the natural oils reaches steady-state at a sliding distance of approximately 2000 m. The experimental results show a clear distinction between the oils where the avocado, olive, canola, and peanut
Natural oils as carrier fluids
Previous studies by the authors have investigated the size effect of boron nitride and boric acid particulate additives mixed into canola oil for their effects on friction and wear [21], [31], [32]. Canola oil was used as a carrier fluid to circulate the particulate additives during the pin-on-disk testing. Here, the canola oil with the particulate additives demonstrated improved tribological performance. However, taking into consideration the presented fatty acid analysis, canola oil was not
Conclusion
Natural oils are non-toxic, renewable, and feasible alternatives to petroleum-based lubricants that can satisfy the combination of environmental, health, economic, and performance challenges of modern lubricants. The current investigation examined the mechanisms governing the improved tribological performance and thermal response of the natural oils. The prevailing trend with the natural oils is that high oleic acid concentrations improve friction and wear performance by establishing densely
References (59)
- et al.
Effects of unsaturation on film structure and friction of fatty acids in a model base oil
J Colloid Interface Sci
(2008) - et al.
Increasing formability in sheet metal stamping operations using environmentally friendly lubricants
J Mater Process Technol
(2006) - et al.
An evaluation of multipurpose oil seed crop for industrial uses (Jatropha curcas L.): a review
Ind Crops Prod
(2008) - et al.
The physicochemical and tribological properties of oleic acid based triester biolubricants
Ind Crops Prod
(2011) - et al.
Oxidation and low temperature stability of vegetable oil-based lubricants
Ind Crops Prod
(2006) - et al.
On the friction and wear performance of boric acid lubricant combinations in extended duration operations
Wear
(2006) Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel)
Fuel Process Technol
(2005)- et al.
Metathesis of methyl soyate with ruthenium catalysts
Fuel
(2006) - et al.
Coconut oil as base oil for industrial lubricants-evaluation and modification of thermal, oxidative and low temperature properties
Tribol Int
(2006) - et al.
Mechanical properties of the hexagonal boron nitride monolayer: ab initio study
Comput Mater Sci
(2012)
Improvement of pour point and oxidative stability of synthetic ester basestocks for biolubricant applications
Arab J Chem
Vegetable oil-based lubricants: a review of oxidation
Tribol Int
Low-temperature flow behaviour of vegetable oil-based lubricants
Ind Crops Prod
Fundamentals of lubrication
Unsaturated fatty acids in alkane solution: adsorption to steel surfaces
Langmuir ACS J Surf Colloids
Influence of boric acid additive size on green lubricant performance
Philos Trans R Soc A Math Phys Eng Sci
Boundary lubrication performance of free fatty acids in sunflower oil
Tribol Lett
Analyses of the adsorption structures of friction modifiers by means of quantitative structure–property relationship method and sum frequency generation spectroscopy
Tribol Online Tribol Online
The influence of temperature on the stability of a mineral oil
Trans Faraday Soc
High oleic plant oils with hydroxy fatty acids for emission reduction
2005 World tribology congress III. Washington, D.C
J Am Chem Soc
Lubricant base stock potential of chemically modified vegetable oils
J Agric Food Chem
Biodegradation of vegetable oils: a review
Sci Res Essays
Biolubricants: raw materials, chemical modifications and environmental benefits
Eur J Lipid Sci Technol
Tribochemical reaction of stearic acid on copper surface studied by surface enhanced Raman spectroscopy
Tribol Trans
American Society for M. ASM metals reference book
Green lubricants: role of additive size
One-pot synthesis of chemically modified vegetable oils
J Agric Food Chem
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